1. Field of the Invention
This invention relates to a method of and an apparatus for detecting the maximum cylinder pressure angle in an internal combustion engine, and more particularly to a method of and an apparatus for detecting the maximum cylinder pressure angle in an internal combustion engine in which the maximum cylinder pressure position is once obtained as a time value and is thereafter converted into an angular value by multiplying by a time-angle conversion factor.
2. Description of the Prior Art
There are known methods or apparatuses for detecting the crankshaft angle of an internal combustion engine at which the cylinder pressure becomes maximum and for controlling the ignition timing or the like of the internal combustion engine on the basis of the detected maximum cylinder pressure angle. For example, Japanese Laid-open Patent Publication No. 58(1983)-197470 discloses an apparatus in which the maximum cylinder pressure angle is determined using a crankshaft angle sensor which detects the crankshaft angle once every unit angle of rotation, for instance, once every degree of rotation. This conventional apparatus thus detects the maximum cylinder pressure angle directly in terms of the crankshaft angle and not by first once determining the position of the crankshaft as a time value. When the maximum pressure angle is obtained directly as a crankshaft angle as in the aforesaid conventional apparatus, it is necessary to employ a sensor capable of accurately detecting the crankshaft angle, for example, once every angle of rotation. This is disadvantageous since it is extremely difficult and very expensive to fabricate a sensor capable of such high-precision measurement.
Therefore, methods or apparatuses have been employed in which the position at which maximum pressure occurs is first measured as a time value and the measured time value is thereafter multiplied by a time-angle conversion factor to convert it into an angle. In this case, the time-angle conversion factor is usually calculated as (engine rpm.times.360 degrees)/60 sec. However, since the engine rotational speed (rpm) varies during acceleration or other transient condition of engine operation, it is necessary to avoid the influence of such variation by instantaneously detecting the engine rpm. The conventional method or apparatus for doing this will be explained with reference to FIG. 5 taking a four-cylinder engine by way of example. In addition to obtaining a cylinder identification signal once every 720 degrees rotation of the crankshaft and a TDC (top dead center) signal once every 180 degrees rotation thereof, unit crankshaft angles .theta.0-.theta.5 are detected once every 30 degrees rotation of the crankshaft. The conversion factor is calculated from the periods ME1-ME6 of the intervals S1-S6 between the unit crankshaft angles .theta.0-.theta.6 (these intervals being referred to as "stages" hereinafter) and, in particular, is calculated from the period ME1 of stage S1, which is the stage during which the maximum pressure Pmax occurs. As will be noted from FIG. 6 relating to one reciprocation of a piston, however, the engine rpm is not constant even during steady-state engine operation but varies as a function of piston position, power stroke pressure and the like. Thus, when the engine rpms Ne0 to Ne5 at angles .theta.0 to .theta.5 are calculated and the rate of change aNE is found as ##EQU1## it is found that the angular velocity varies as shown in FIG. 7. Therefore, in calculating the time-angle conversion factor, even if the calculation is carried out using the period ME1 of the stage S1 in which the maximum pressure value occurs, the calculated angular step will be erroneous since the period ME1 represents nothing more than the average rpm value within stage S1. In other words, as shown in FIG. 8, where the actual crankshaft angle is 13 degrees ATDC (after top dead center), the value of .theta.pmax calculated from ME1 will be 14 degrees ATDC, i.e. will be larger than the actual value. This error in the direction of retardation will result in an error in the control of the ignition timing or the like, which in turn will lower the engine operating efficiency.